Age calculator
Calculate exact age in years, months and days.
Fill in the fields and the result will appear here automatically.
Use this age calculator to find exact age on today or another target date.
How it works
Formula and logic
Age is decomposed forward from the birth date: take the largest number of complete calendar months that fits before the reference date, then count the remaining calendar days. Complete months divide into years and a month remainder. If an anniversary month lacks the original day, its last day is used. Thus 31 January to 28 February is a complete month; a 29 February birth has a 28 February anniversary in a common year. This is this model’s convention, not a legal birthday rule. Total days compare calendar dates without elapsed hours or daylight-saving shifts. A reference date before birth is rejected. An empty reference date uses the device’s current date; enter both dates explicitly for a reproducible calculation. Dates use YYYY-MM-DD and years 0001–9999 under the proleptic Gregorian calendar, without country-specific historical changeovers.
Example
From 1 January 1990 to 1 February 2026 the model gives 36 years,1 month and 0 days.
Fields and units
- Date of birth — date. Use YYYY-MM-DD, years 0001–9999, in the proleptic Gregorian calendar.
- Target date — date. Blank means today on this device; an explicit date makes the result reproducible.
How to use
- — Enter date of birth.
- — Choose target date if needed.
- — Review exact age and total days.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- NASA, section 2.1: Gregorian leap rule and proleptic dates; anniversary and shift conventions belong to this model
- Limitation
- If an anniversary month lacks the original day, its last day is used. This is this model’s convention, not a legal birthday rule.
FAQ
Which dates are compared for age?
Enter the date of birth and optionally a reference date. Without a reference date, the calculator uses today.
How are years, months and days counted?
The calculation compares calendar dates, accounting for different month lengths and leap years rather than dividing elapsed milliseconds by 24 hours.
What does total lived days mean?
It is the count of calendar days from the birth date to the reference date, independent of daylight-saving clock changes.
How is a 29 February birthday treated in a common year?
This model uses 28 February when the anniversary month lacks day 29. That convention helps reproduce the arithmetic; it does not establish a legal birthday or an official age threshold.